Various proposals for a protection circuit against Electrostatic Discharge (ESD) have been made heretofore. ESD means a discharge from an electrically charged body or machine to a semiconductor device and a discharge from an electrically charged semiconductor device to the ground potential. When an ESD occurs in a semiconductor device, a large amount of electric charge from the terminal flows into the semiconductor device as a current and the electric charge generates a high voltage inside the semiconductor device, hence causing, for example, a dielectric breakdown of the internal elements and subsequently a fault in the semiconductor device.
A representative example of the electrostatic protection circuit is an RC triggered (RCT) MOS circuit. The RCT MOS circuit uses a trigger circuit comprising a serial circuit including a resistor and a capacitor serially connected between power terminals. A voltage at the connection point between the resistor and the capacitor is set as a trigger signal for driving a shunt transistor for discharging the electrostatic surge. By turning on the shunt transistor, a discharge current of an ESD surge flows in a power line and a high voltage may be applied to the internal circuit also connected to the power line. It is desirable to avoid breaking the internal circuit as a result of a high voltage being applied due to the discharge of the ESD surge.